An indexed file is a mass storage file in which data records can be accessed by the value of a key. A record description can include one or more key data items, each of which is associated with an index. Each index provides a logical path to the data records, according to the contents of a data item within each record which is the record key for that index.
The data item named in the RECORD KEY clause of the file control entry for a file is the prime record key for that file. For purposes of inserting, updating and deleting records in a file, each record is identified solely by the value of its prime record key. This value should, therefore, be unique and must not be changed when updating the record.
The data item named in the ALTERNATE RECORD KEY clause of the file control entry for a file is an alternative record key for that file. The value of an alternative record key can be non-unique if the DUPLICATES phrase is specified. These keys provide alternative access paths for retrieval of records from the file.
Your COBOL system provides an extension which allows the key field to be a split key. A split key is a key comprising two or more data items, which may or may not be contiguous, in the record description.